There are two basic classes of adhesives in widespread current use. The first class is pressure sensitive adhesives, such as are employed in adhesive tapes. The adhesive is tacky, and bonding takes place primarily due to physical forces of attraction between the adhesive and the substrate surface, when the adhesive is pressed onto a surface. In this case, adhesion is temporary and, if sufficient force is applied, the adhesive can be removed from the substrate. A second class of adhesives, used primarily for structural purposes, is reactive adhesives. During the establishment of the adhesive bond, a reaction takes place, within the adhesive itself and with the substrate(s) to be bonded, that involves the formation of permanent covalent chemical bonds. Such adhesives are more or less permanent, and removal of the adhesive cannot occur except through disruption of the chemical bonds.
Epoxy structural adhesives are reactive adhesives that are widely used in industrial applications. They provide the best combination of thermal resistance, chemical inertness, adhesive bond strength and mechanical characteristics of all known types of adhesives. For this reason, they are widely employed in the aerospace, transportation and electronic industries. Epoxy adhesives also find applications in the joining and bonding of composites, piping and in many construction adhesives employed in the construction, housing and civil engineering industries. The reliability and ease of fabrication utilizing epoxy adhesives have virtually revolutionized these industries and have rendered conventional types of mechanical fasteners (i.e. bolts, screws, nails, etc.) obsolete in many applications. In many cases, the mechanical properties of adhesively bonded joints exceed those made with conventional fasteners. An even greater use of adhesives would be realized if these materials were more convenient to use and underwent faster bonding.
The trade off between working time of a curing system and its ultimate cure time is one of the classic epoxy formulating challenges. Obviously, it is desirable to have plenty of working time at room temperature to complete assembly operations—especially if valuable components are involved. Once assembly is complete, rapid completion of cure is desired so that the next step in the operation can be started. Unfortunately, when these requirements are translated into polymer science terms, a conflict arises. The cure rate of most epoxy systems follows an Arrhenius type relationship wherein the log of cure rate is inversely proportional to the reciprocal of absolute temperature. For many practical applications, process engineers have used the rule of thumb that cure rate will double with each 10° C. increase in the process temperature.
To achieve both good working life and quick process time, a difference in cure rate of about three orders of magnitude is required between the assembly and cure conditions. By the rule of thumb this estimates to roughly 100 degrees difference between assembly and cure temperatures. For this reason, it is typical to find epoxy systems that are assembled at room temperature and cured at 120° C. Curing temperatures of 150 and 180° C. are also common and provide an even greater difference between assembly and curing reaction rates. If one attempts to further reduce the difference between the processing and cure temperatures, the systems become unworkable, because the processing time (i.e. potlife) is correspondingly shortened. In addition, since under these conditions the curing is progressing during the assembly, the characteristics of the adhesive are also constantly changing—making application difficult, unreliable and irreproducible. Ever since the discovery of epoxy resins in the mid 1940's there has been a search for ways to avoid this dilemma.
In many applications subjecting the substrate(s) to be bonded to high temperatures cannot be tolerated, or it is very inconvenient. Therefore, a number of options have been developed for room temperature or a slightly elevated temperature cure. These will be reviewed briefly as possible solutions:
Frozen Systems—In this approach, a highly reactive epoxy curing system is rapidly mixed at room temperature, filled into an application package and then dropped into a dry ice box or similar cold storage. During assembly, the package is thawed and applied. Cure then proceeds at room temperature. This approach avoids the necessity of mixing the adhesive formulation directly at the work site and provides a reasonably fast cure time (eg. 6-8 hours). The disadvantage of such systems is that since the handling characteristics of the adhesive constantly change as the adhesive thaws, it is difficult to automate the application and cure cycles. In addition, once thawed, cure starts spontaneously and the entire batch must be used in a relatively short time. Any excess is wasted and cannot be recovered.
Blocked Catalyst Systems—A number of ingenious systems have been developed to allow incorporation of a highly reactive catalyst system into a resin while blocking its reactivity at room temperature. One of the first systems encapsulated the catalyst inside a wax with a sharp melting point temperature. Below the melting point the system has excellent working life, but it cures extremely rapidly once the melting point is reached. Catalysts have also been blocked by the use of a chemical adduct with a sharp decomposition point temperature. Employing blocked catalysts allows formulation of systems where only about 60° C. difference is required between assembly and final cure temperature. Blocked catalysts have not found much general usage since they tend to be “tempermental” with respect to their cure characteristics and they are subject to spontaneous curing during storage.
Surface Activated Systems—In those instances where polymer systems are cured in close contact with surfaces, a number of novel cure systems have proven effective in supplying good working life followed by rapid room temperature cure. The most common examples are anaerobic and cyanoacrylate adhesives. These systems possess excellent processing characteristics provided the gap between the component surfaces is less than 0.5 mm. Unfortunately, these same adhesives also have poor thermal resistance and cannot be used at even modest temperatures (eg. 100° C.).
Cure-on-Command Systems—These systems typically have very long shelf and working life, but cure rapidly when the assembly operator or an automated assembly system gives the “command”. The command usually is the application of actinic radiation to the polymer system being cured. The earliest examples used ultraviolet light, but UV irradiation can only be used only when at least one of the substrates is transparent to light. The cure of opaque, filled or thick cross sections is problematic using UV triggered systems.
“Dual-cure” systems have also been described. They employ both light and heat to achieve cure of epoxy and other types of monomers and they are used in applications in which it is not possible to directly irradiate the material. The dual-curable system is first applied to the substrate and irradiated to form a gelled shell. The gelling process (i.e. a functionally significant increase in viscosity) gives rise to certain problems of its own, and it would be advantageous to avoid gelling if one could. The material, which is usually located such that it cannot be cured by light, is subjected to a thermal treatment designed to trigger the initiator by heat. In some cases, the material is first subjected to low level UV irradiation on one adherand and then very rapidly covered with the other adherand to make an adhesive joint. These latter materials are characterized by a very short “open time”. This means that the time from completion of the irradiation to when an adhesive bond may be made is very short, on the order of a few seconds.
As described above, although many schemes have been advanced, at present there appears to be no workable solution to the dilemma of obtaining simultaneously both a long working life and a rapid cure time. In principle the notion of a Command-Cure involves two seemingly diametrically opposed requirements. The first of these requirements is a latent system that is stable for indefinite periods at room temperature. Such systems would, therefore, be said to be storage stable. The second requirement is high reactivity at that would allow the system to undergo cure (polymerization) only when desired and with minimal exposure to heat.